can only be determined by accurate visual observations made under favourable atmospheric conditions.
Comparing the photographs with the map constructed by the Committee of the British Association, and published in their report for 1872, page 247, we find no difficulty in identifying the more prominent markings, and we see that no very important change can have occurred during that period.
A brief account of Messier and Messier A has already been given upon page 49. It was there shown that sometimes one of them is the larger and sometimes the other. The craters themselves vary in shape, sometimes one being triangular and the other elliptical, and sometimes vice versa. If both happen to be elliptical at the same time, their major axes are sometimes parallel and sometimes nearly at right angles. Some of these changes and some others are illustrated by drawings (see Plate D, Figs, 1 to 5).
Since the Jamaica photographs were taken it appeared that it would be a matter of interest to illustrate these changes by means of photography. While some of them can be detected by a careful examination of the plates at the end of this volume, yet it was not possible to publish the atlas on a sufficiently large scale to show all the detail of which the negatives are capable; and the atlas labors under a certain disadvantage on this account. It, therefore, seemed best to select six negatives of these craters, not necessarily those giving the best definition, but those which showed the changes under investigation to the best advantage. These negatives have all been enlarged between four and five times, to the same scale as that of Plato. On this scale the Moon would be five feet nine inches in diameter. The advantages of enlargement are clearly shown if we compare any of the six figures of Messier on the adjoining plate with the unenlarged plates of the atlas taken from the same original negatives. In the case, for instance, of Plates 2A [3.2, 1.2] and 2C [3.2, 1.8], a comparison with Figures 3 and 6 of Plate G will show that a great deal of detail is sacrificed when it becomes necessary to publish on a small scale.
Messier also appears upon Plates 21 and 27 of the Paris charts. At the time these plates were taken the colongitude of the terminator was practically the same as in Figures 3 and 4. In "Weinek's Atlas" Messier is shown on Plates 57 and 58. As this atlas gives only sunrise and sunset views, the more interesting features of these craters are not shown.
In the table the first column gives the number of the figure on Plate G, the second the corresponding plate of the atlas, the third and fourth the date and Greenwich
